There is a known conventional image forming apparatus which performs booklet printing. In booklet printing, image for two pages are printed on each front and back side of a plurality of sheets. The plurality of sheets printed as such are stacked, folded into two, and bound to create a booklet.
As shown in FIG. 9, image data for four pages printed on one recording sheet are not in serial page order. Thus, it is necessary to sort and combine the image data before printing. In FIG. 9, N represents a total number of prints.
Accordingly, a commonly used image forming apparatus stores all the image data (i.e., image data for all the pages of one booklet) subject to booklet printing once in memory. Then, sorting and combining the image data are started.